Flame treatment of polymeric film and apparatus



Aug. 18, 1964 w. L. BRYAN 3,145,242

FLAME TREATMENT OF POLYMERIC FILM AND APPARATUS Filed Sept. 24, 1962 GASEOUS SUPPLY T0 JET DELIVERY NOZZLE; AIR NITRUGEII, ETC.

. v I Am l8b SENSING ||0ZZLE MR SUPPLY INVENTOR WILLIAM LOCKE BRYAN BY /m/%;L I

ATTORNEY United States Patent 0 3,145,242 FLAME TREATMENT OF POLYMERIC FILM AND APPARATUS William Locke Bryan, Tonawanda, N.Y., assignor to E. L

du Pont de Nemours and Company, Wilmington, Del, a

corporation of Delaware Filed Sept. 24, 1962, Ser. No. 225,639 6 Claims. (Cl. 264-84 This invention relates to the treatment of plastic surfaces and more particularly to improvements in the process and apparatus for flame treating the surface of continuous webs of organic polymeric films to modify the surface characteristics thereof.

When flame treating relatively thin polymeric sheets and films in order to modify their surface properties, it is common practice to employ an elongated burner disposed transversely of the direction of travel of the film through the treating apparatus and to maintain the film, during its traverse of the flame, in contact with a relatively highly thermally conductive surface positioned opposite the flame and maintained at a temperature sufficiently below that of the fiame to preclude degradation of the bulk of the polymeric film. The burner is usually longer than the width of the widest film to be treated. Consequently the edges of the film are exposed to (are seen by) more flame in proportion to the mass of film than are the regions of the film between the two edges. This results in the edges of the film becoming hotter than the rest of the width which causes them to curl and thicken, in some cases a distinct bead being formed. In some cases the distortion caused by this localized overheating results in as much as a quarter inch of film folded or rolled over at each edge of the web width. When recovering such a treated film by winding on a conventional reel, the thicker edges result in a poor wind-up having a greater diameter at the ends of the roll (i.e. the edges of the film) than at the middle. In addition, with oriented films, localized overheating of the edges tends to cause edge lane shrinkage with the result that the film is distorted on winding up and poor sheet flatness results. The expedient of continuously slitting and removing a narrow trim from each edge of the film is so obviously wasteful as to be an economically unacceptable solution to the problem.

It has been proposed to avoid overheating at the edges of polyethylene film} or flattened polyethylene tubing undergoing flame treatment by suitably reducing the longitudinal dimensions of the flame through the agency of cover plates which may be adjusted to cover and thus block out predetermined areas of the burner opening. However, whenever the area of burner opening is altered, the gaseous mixture fed to the burner and the pressure must also be suitably altered in order to maintain the desired flame characteristics. It will be obvious that the use of cover plates to regulate the longitudinal dimension of the flame in order to provide either untreated margins or to avoid overheated edges depends for success on the film or flattened tubing being treated being of substantially unvarying width and tracking perfectly through the treating apparatus. This ideal situation is rarely realized in actual practice, it being more commonly experienced that the positions of the edges of the polymeric webs will vary rather randomly as a typical mill roll of production film or tubing unwinds.

It is therefore an object of this invention to provide an improved method for avoiding edge overheating in the flame treatment of a polymeric web and for providing, if desired, an untreated margin or lane on said web, which method does not require the adjustment of either the pressure or the composition of the fuel mixture supplied to the burner, and which is readily adapted to accommodate the above-described edge-wandering of the film unwinding from the supply roll. A further object is to provide an apparatus for carrying out the above method. The foregoing and related objects will more clearly appear from the detailed description which follows.

These objects are realized by the present invention which, briefly stated, comprises, in a process and apparatus for flame treating a polymeric film wherein continuous organic polymeric film is brought continuously into intimate contact with a relatively highly thermally conductive surface and while in contact therewith passes through the flame of an elongated burner spaced from and disposed parallel to said film and transversely of the direction of travel thereof, whereby to heat the surface of said film exposed to said flame to an elevated temperature, said thermally conductive surface opposite said flame being maintained at a temperature substantially below said elevated temperature; the improvement which comprises the step of, and adjustable means for, directing a narrow stream of substantially inert gas through the gap between said film and said burner in the vicinity of each edge of said film and in a direction substantially paralleling the direction of travel thereof through said flame, whereby to cool said film in the vicinity of the edges thereof.

I have found that the hereinbefore described problems attending the flame treatment of polymeric films can be eliminated by directing narrow streams or jets of a substantially inert gas through the gap between the film and the burner in the vicinity of the edges of the film and in a direction substantially parallel to the direction of its travel through the treating apparatus. To the extent that these jets actually impinge on the film, they provide a localized cooling effect augmenting the heat quenching capacity of the supporting relatively highly thermally conductive surface in preventing or at least minimizing the chance of overheating the edges of the film. To the extent that these jets actually impinge on the flame, they provide at least a localized distortion of the flame, pushing it away from the film and thereby reducing the heat flux to the film in the vicinity of its edges. When the cross section of the jets passing through the gap between film and burner is sufficiently large in proportion to the width of the gap, even relatively low jet velocities will completely extinguish a narrow portion of the flame in the vicinity of each film edge. This accomplishes a major reduction in the heat flux to the film while the jet itself provides some small additional cooling effect. Consequently, it is preferred to direct the gaseous jets in such a way as to extinmiish narrow portions of the flame.

Any gas may be used for the jets provided it is substantially inert. For example, gases which do not comprise combustibles (for example, fuel components such as hydrocarbons, hydrogen, carbon monoxide) or which of themselves will not tend to cause local increases in the burning velocity of the flame (as would pure oxygen) may be employed. Among gases found useful may be mentioned air, nitrogen, helium and carbon dioxide. Gases generally considered unsuitable for this purpose either for safety or economic reasons include hydrogen, carbon monoxide, oxygen and gaseous paraflinic and olefinic hydrocarbons.

My invention will now be more particularly described in connection with the accompanying drawing wherein:

FIG. 1 is a schematic illustration of an arrangement of apparatus suitable for carrying out the process of this invention; and

FIG. 2 is a schematic illustration, partly in section, showing in detail a preferred film edge-sensing and following apparatus.

Referring to FIG. 1, film F to be flame treated un-- winds under substantially constant tension from supply roll 1 and is conducted through a train of rolls consisting of a metal idler roll 2, a driven rubbercovered tension isolation roll 3 and metal idler roll 4. From there the film substantialy reverses its direction of travel by passing around idling rubber-covered nipping roll 5 which brings it into contact with a relatively highly thermally conductive surface, in this case, rotating metal treating back-up roll 6 which is equipped in conventional manner (not shown) for the passage therethrough of heat transfer fluid (usually water). Idling rubber-covered nipping roll 5 serves to minimize wrinkling and press out entrapped air which substantially improves the uniformity of contact of the film with back-up roll 6 as it passes burner 7. After the treated film leaves back-up roll 6 it passes under metal idler roll 8, over driven metal tension isolation roll 9 and finally under metal idler roll 10 to driven wind-up roll 11.

Burners varying widely in design can be and are commonly employed in flame treating a wide variety of polymeric films and sheeting. Elongated burners whose discharge openings permit the establishing and maintaining of a substantially uniform flame front are preferred. Included among such burners are those whose discharge openings consist of one or more longitudinally disposed rectangular orifices defined preferably by adjustable lips and those whose discharge openings consist of multiple longitudinally disposed rows of holes the size and relative spacing of which is selected to contribute to flame front uniformity. Flame fronts measuring (in the direction of film travel) from less than flt-inch up to several inches at film-to-burner distances ranging from less than /s-inch up to nearly an inch are commonly employed in carrying out the flame treatments in conjunction with which the invention herein described may be used.

A jet delivery nozzle and film edge-sensing and following means for contiuously automatically positioning the jet delivery nozzle in response to changes in the lateral position of the edge of the film are shown schematically in FIG. 1 and in more detail in FIG. 2. Jet delivery nozzle-positioning cylinder 12 (filled with hydraulic fluid) is suspended from traversing bar 13 by means of carriage 19, provided with a pair of cylindrical wheels 20 which roll in a rectangular slot (not shown) milled into the upper quadrant of traversing bar 13 which extends transversely across the entire width of back-up roll 6. Locking screw 21 permits the lateral position of carriage 19 on bar 13 to be fixed. Jet delivery nozzle 14 is supported by arm 22 which in turn is secured to and supported by one end of the center rod of hydraulically positionable piston 23. The other end of the center rod of piston 23 is secured to and supports yoke 24 which in turn supports edge-sensing nozzle 15, of the slotted orifice type. A pneumatic pressure signal from the upper jaw of edge-sensing nozzle 15 passes through signal line 16 to controller 17, a pneumatic-hydraulic transducer of conventional design. If the amplitude of the pressure signal varies from a pre-set value (as it would if the position of the edge of the film varied laterally from a predetermined position), the controller acts (through a pair of hydraulic lines 18) to establish a differential hydraulic pressure across the piston in cylinder 12 of such magnitude and direction that the piston (and thereby edgesensing nozzle 15) is moved to a position which restores the pneumatic signal to its preset value, Thus the position of jet delivery nozzle 14 relative to the edge of the film remains substantially unchanged although the edge of the film may wander laterally over a considerable distance as it passes through the flame treating apparatus. Two such assemblies are needed for continuously and automatically positioning a jet delivery nozzle relative to each edge of the traveling film.

In operation, after the flame treating apparatus has been threaded up with the film to be treated, piston 23 is centered in cylinder 12 and carriage 19 is then moved along traversing bar 13 until jet delivery nozzle 14 is in the position relative to the edge of film F (supported on treating back-up roll 6) shown in the drawing. When the jet delivery nozzle 14 is in this position, the film in the span between rolls 4 and 5 of FIG. 1 passes through the U-shaped opening of edge-sensing nozzle 15, approximately mid-way between the upper and lower jaw thereof and with its edge at about the mid-point of the length of the jaws. This is the pre-set condition referred to in the previous description relating to FIGS. 1 and 2. Air is continuously supplied to the lower jaw of edge-sensing nozzle 15. If the film passing between the jaws of edgesensing nozzle 15 should move to the right of the pre-set condition, the amplitude of the pneumatic pressure signal transmitted via signal line 16 to controller 17 (a pneumatic-hydraulic transducer of conventional design) would decrease, causing the controller to act to increase the hydraulic pressure in line 18a and proportionately to decrease the hydraulic pressure in line 18b. This will create an hydraulic pressure differential across piston 23 in cylinder 12 which will move piston 23 to the right until edge-sensing nozzle 15 has resumed its pre-set position relative to the edge of the film and the amplitude of the pneumatic pressure signal has been restored to the pre-set evel. Conversely, if the film passing through edge-sensing nozzle 15 should move to the left, the amplitude of the pneumatic signal transmitted via signal line 16 would increase, causing controller 17 to act to decrease the hydraulic pressure in line 18a and to increase the hydraulic pressure in line 18b, establishing in cylinder 12 an hydraulic pressure differential across piston 23 of opposite direction to that hereinabove described. This pressure differential would move piston 23 to the left until the edge-sensing nozzle 15 had resumed its pre-set position relative to the edge of the film and the amplitude of the pneumatic pressure signal had been restored to the pre-set level.

Although the apparatus has been shown as sensing the positions of the edges of the film prior to its passage across the flame front of the burner, it will be understood that the edge-sensing may take place after the film has left the treating back-up roll. For best results the positions of the edges of the film should be sensed a relatively short film-travel distance away from the film-to-burner It should also be understood that the invention may be carried out without the use of the preferred edge-sensing and following nozzle assembly. For example, the jet delivery nozzles may be suspended from a slotted traversing bar by means of appropriately keyed bushings, the lateral positions of the bushings being manually variable through the action of hand-cranked worm gears.

In FIG. 1, burner 7 is shown in the position relative to treating back-up roll 6 which is preferred from the standpoint of minimizing thermally caused distortion of the burner. Delivery nozzles 14 are shown positioned below the burner and aimed upward to direct their gaseous jets through the aperture between film and burner in a direction substantially paralleling the direction of film travel through the treating apparatus. Although the delivery nozzles could be positioned above the burner, aimng downward, the former positioning is preferred since it greatly reduces the heat flux from the burner to the nozzles and thus minimizes the risk that thermal distortion of either nozzle might misdirect the gaseous jet therefrom. The burner may alternatively be positioned above the treating back-up roll directing its flame downward or below the treating back-up roll directing its flame upward. In either of these alternative apparatus arrangements, delivery nozzles 14 will be positioned to direct their gaseous jets in a substantially horizontal direction either in the direction of film travel or counter thereto.

The jet delivery nozzles employed in carrying out the process of this invention need not be of elaborate design. For instance, successfully employed jet delivery nozzles have been fabricated from 'short lengths of copper tubing ranging in OD. (outside diameter) from fii-inch down to -inch. The deformability of copper tubing permits the cross sectional shape of the gaseous jet to be controlled to some extent by pinching or otherwise shaping the copper tubing. 7

In addition to modifying the cross sectional shape of the gaseous jets by shaping the delivery nozzles, the tips of these nozzles may be and are often skewed a few degrees from being absolutely parallel to the direction of film travel so that they direct their jets slightly away from the center line of the traveling film. This reduces the chance that the expanding gaseous jets will distort or extinguish a portion of the flame front overlapping the edge of the film and thus prevent the proper treatment of the film in these regions. For this reason the skewing of the jet delivery nozzles in this manner is preferred when carrying out the process of this invention.

The jet velocities necessary to accomplish the amount of edge cooling required to prevent curling, beading, or other distortion attributable to local overheating (through the combined effects of gaseous cooling, and/or flame distortion or extinguishment) will normally be determined by trial and error by a skilled operator inasmuch as they depend at least in part on the thickness and nature of the film being treated, its speed through the treating apparatus, the heat quenching capacity of the treating back-up surface in proportion to the heat flux from the flame front, the dimensional stability of the film (particularly if it is biaxially oriented), the width of the flame front measured in the direction of film travel, the film-to-burner distance and both the composition and supply rate of the gaseous mixture with which the burner is fueled.

A wide variety of both chemically and physically dissimilar polymeric films have been successfully flame treated while operating the process of this invention at film speeds ranging from 20 to 500 feet per minute wherein gas rates (from each delivery nozzle) were varied over the range of from about 0.05 to 0.75 standard cubic feet per minute.

The followingexamples will serve to further illustrate the principles and practice of this invention.

Examples 1 to 4 Biaxially oriented films (1.5 mils thick and 42 inches wide) of (l) polyvinyl fluoride, (2) polyvinyl chloride, (3) isotactic linear polypropylene and (4) high density, linear polyethylene were flame treated in the apparatus shown in FIG. 1, using a 53-inch long burner having a longitudinally disposed rectangular discharge orifice 100- mils wide. The burner Was positioned 2.5 millimeters from the film in each case and was supplied with a stoichiometric mixture of air, propane and oxygen at the rate of 15.6 standard cubic feet per minute. A pair of jet delivery nozzles made of 41-inch O.D. copper tubing were mounted in bushings appropriately keyed to move lengthwise of the slotted traversing bar, propelled by handcranked worm gears. Each jet nozzle was positioned relative to an edge of the film in the manner shown in FIG. 2, skewed slightly away from the center line of the film and was caused to follow the lateral edge-wandering of the film by hand cranking each worm gear when necessary. The temperature of the treating back-up roll opposite the burner was maintained in the range of 6575 C. During each run (both control and example), film speed through the treating apparatus was varied from 100 to 400 feet per minute. During the control runs, no gas was supplied to the jet delivery nozzles and curling, thickening and beading of the edges of the film (with consequent distortion of wind-up) occurred to an objectionable degree. During each example run, nitrogen was supplied to one and air to the other jet delivery nozzle at rates (for each nozzle) which Were purposely varied over the range of 0.2 to 0.7 standard cubic feet per minute. In each instance, edge curling, thickening and beading were 6 substantially eliminated even at the lower gas rates within this range. At rates above 0.4 standard cubic feet per minute, the flame was extinguished over a narrow portion of its length (approximately /2 inch) immediately outboard of each edge of the film.

Examples 5 to 8 The runs of Examples 1 to 4, including their corresponding controls were repeated using the automatic film edge-sensing and following apparatus shown in FIG. 2, operating to automatically position a pair of jet delivery nozzles fabricated from Vs -inch O.D. copper tubing. Film speed during each control and example run was varied over the same range as before. During each example run, nitrogen was supplied to one and air to the other jet delivery nozzle at rates (for each nozzle) which were purposely varied over the range of 0.05 to 0.5 standard cubic feet per minute. Edge curling, thickening and beading were substantially eliminated even at the lower gas rates within this range. At rates above about 0.15 standard cubic feet per minute, the flame was extinguished over a narrow portion of its length (approximately 41 inch) immediately outboard of each edge of the film. By contrast, during the control runs when no gas was supplied to either jet delivery nozzle, edge curling, thickening and beading with consequent film distortion were noticeable to an objectionable degree.

Examples 9 and 10 A biaxially oriented polyvinyl fluoride fihn and a tubu-' lar melt-blown, low density branched polyethylene film (each 42 inches wide and 1.5 mils thick) were flame treated in the apparatus shown in FIG. 1, using a 53-inch long burner the discharge opening of which comprised six longitudinal rows of -inch diameter holes spaced on As-inch centers in an equilateral triangular pattern. The burner was positioned 5.0 millimeters from the film in each case and was supplied with a propane:air mixture in the ratio of 1221.6 at the rate of 12.6 standard cubic feet per minute. The automatic film edge-sensing and following apparatus and Ai-inch O.D. jet delivery nozzlesof Examples 5 to 8 were employed. Film speed during each control and example was varied from 30 to 300 feet per minute. The temperature of the treating back-up roll opposite the burner was maintained in the range of 45- 55 C. During the control runs, no gas was supplied to the jet delivery nozzles and curling, thickening and. beading of the edges of the film (with consequent distortion of wind-up) were noticeable to an objectionable degree. During each example run, nitrogen was supplied to one and helium to the other jet delivery nozzle at rates (for each nozzle) which were purposely varied over the range of 0.1 to 0.7 standard cubic feet per minute. In contrast to the control runs, edge curling, thickening and beading were substantially eliminated even at the lower gas rates within this range. At rates above about 0.5 standard cubic feet per minute, the flame was extinguished over a narrow portion of its length (approximately inch) immediately outboard of each edge of the film.

It will be evident from the foregoing description and. examples that this invention provides an economical and easily operated technique for eliminating film edge distortion caused by overheating during flame treatment. It also makes available a means for providing one or more untreated margins or lanes on the film being treated. In contrast to solutions suggested by the prior art, since the gaseous jets of the invention at hand do not obstruct the flow of gaseous fuel from the burner orifices, there. is no need to adjust either the composition of the fuel mixture or its rate of supply to the burner in order to maintain a stable flame. In addition, the use of gaseous jets is readily adapted to cooperating with an automatic film edge-sensing and following apparatus of conventional design, an advantage not attributable to the prior art technique.

I claim:

1. In a process for flame treating a polymeric film wherein continuous film of organic polymeric material is brought continuously into intimate contact with a relatively highly thermally conductive surface and while incontact therewith passes through the flame of an elongated burner spaced from and disposed parallel to said film and transversely to the direction of travel thereof whereby to heat the surface of said film exposed to said flame to an elevated temperature, said thermally conductive surface opposite said flame being maintained at a temperature substantially below said elevated temperature; the improvement which comprises, in combination, the step of directing a narrow stream of substantially inert gas through the gap between said film and said burner in the vicinity of the edge of said film and in a direction substantially paralleling the direction of travel thereof through said flame, whereby to cool said film in the vicinity of the edge thereof.

2. The process of claim 1 wherein said stream of inert gas is effective to extinguish that portion of the flame in the path of said stream.

3. The process of claim 1 wherein said stream of inert gas is moved laterally responsive to variations in the path of travel of the edge of the film through said flame whereby to maintain said stream in fixed position with respect to the edge of said film.

4. In apparatus for continuously flame treating continuous film of organic polymeric material comprising in combination a relatively highly thermally conductive surface adapted to support continuously travelling continuous film, an elongated burner spaced from said surface and disposed parallel to said surface and transversely of the direction of travel of film thereon said burner being of a length at least that of the maximum width of film travelling over said surface, means for continuously supplying a fuel mixture to said burner whereby to sustain a stable flame directed across said surface and effective to heat film travelling over said surface to an elevated temperature, means to maintain said surface opposite said burner at a temperature substantially below said ele* vated temperature, and means for continuously bringing continuous film into intimate contact with said surface and passing through said flame; the improvement which comprises, in combination, means for directing a narrow stream of substantially inert gas through the gap formed between said burner and said surface, in vicinity of the edge of film passing over said surface, and in a direction substantially paralleling the direction of travel of said film through said flame.

5. In apparatus for continuously flame treating continuous film of organic polymeric material comprising in combination a relatively highly thermally conductive surface adapted to support continuously travelling continuous film, an elongated burner spaced from said surface and disposed parallel to said surface and transversely of the direction of travel of film thereon said burner being of a length at least that of the maximum width of film travelling over said surface, means for continuously supplying a fuel mixture to said burner whereby to sustain a stable flame directed across said surface and effective to heat film travelling over said surface to an elevated;

temperature, means to maintain said surface opposite said burner at a temperature substantially below said elevated temperature, and means for continuously bringing continuous film into intimate contact with said surface and passing through said flame; the improvement which comprises, in combination, means for directing a narrow stream of substantially inert gas through the gap formed between said burner and said surface, in vicinity of the edge of film passing over said surface, and in a direction substantially paralleling the direction of travel through said flame, and means for maintaining said stream in predetermined position relative to said edge of the travelling film.

6. The appartus of claim 5 wherein said means for maintaining said stream in predetermined position relative to said film edge is responsive to deviations in the path of travel of said film edge.

References Cited in the file of this patent UNITED STATES PATENTS 2,648,097 Kritchever Aug. 11, 1953.

2,683,894 Kritchever July 20, 1954 2,746,084 Kreidl May 22, 1956.

2,795,820 Grow et al. June 18, 1957 FOREIGN PATENTS 827,195 Great Britain Feb. 3. 1960 

1. IN A PROCES FOR FLAME TREATING A POLYMERIC FILM WHEREIN CONTINUOUS FILM OR ORGANIC POLYMERIC MATERIAL IS BROUGHT CONTINUOUSLY INTO INTIMATE CONTACT WITH A RELATIVELY HIGHLY THERMALLY CONDUCTIVE SURFACE AND WHILE INCONTACT THEREWITH PASSES THROUGH THE FLAME OF AN ELONGATED BURNER SPACED FROM AND DISPOSED PARALLEL TO SAID FILM AND TRANSVERSELY TO THE DIRECTION OF TRAVEL THEREOF WHEREBY TO HEAT THE SURFACE OF SAID FILM EXPOSED TO SAID FLAME TO AN ELEVATED TEMPERATURE, SAID THERMALLY CONDUCTIVE SURFACE OPPOSITE SAID FLAME BEING MAINTAINED AT A TEMPERATURE SUBSTANTIALLY BELOW SAID ELEVATED TEMPERTURE; THE IMPROVEMENT WHICH COMPRISES, IN COMBINATION, THE STEP OF DIRECTING A NARROW STREAM OF SUBSTANTIALLY INERT GAS THROUGH THE GAP BETWEEN SAID FILM AND SAID BURNER IN THE VICINITY OF THE EDGE OF SIAD FILM AND IN A DIRECTION SUBSTANTIALLY PARALLELING THE DIRECTION OF TRAVEL THEREOF THROUGH SAID FLAME, WHEREBY TO COOL SAID FILM IN THE VICINITY OF THE EDGE THEREOF. 